Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

Implying higher distortion in bridged mode for the AHB2 is incorrect.

It’s all in the measurements and how they’re presented, just look.

Distortion graphs

Here is the graph NOT BRIDGED 8ohms
https://www.stereophile.com/images/1115BAHB2fig04.jpg

Here is the graph NOT BRIDGED 4ohms
https://www.stereophile.com/images/1115BAHB2fig05.jpg

Here is theStereophile graph BRIDGED 8ohms
https://www.stereophile.com/images/1115BAHB2fig06.jpg

They are very similar yes? what do you think will happen if another measurement was done at 4ohm in the bridged mode????? It will be far worse than the non bridged into 4ohms or 8ohms, that’s why they didn’t give the graph even though they probably did measure it.

Cheers George

@george

I imagine in the 4 ohm bridged mode the current limiting would become apparent. JA is often kind by omission. I think the amp would shut down with a 4 ohm bridged load. Thats like 2 ohms per channel as you well know. 

I am just amazed at how well Benchmark marketing has pulled off selling 2x amplifiers.

Now that I am sucked into this I want to study their white papers for veracity.  I understand there is some power supply wonderfullness going on. Feedforward is always amusing too. What a world we live in. 

I imagine in the 4 ohm bridged mode the current limiting would become apparent. JA is often kind by omission. I think the amp would shut down with a 4 ohm bridged load. Thats like 2 ohms per channel as you well know.

Yes and before it shut down, the distortion would be higher than the 8ohm they did give.

I am just amazed at how well Benchmark marketing has pulled off selling 2x amplifiers.

Great marketing, all in the wording and the omission of certain measurement graphs.

I understand there is some power supply wonderfullness going on.

Yes, like Krells Plateau biasing except done on the voltage rails instead,
Quad 405 I think used something similar, and maybe Devialet Class-D amps do something similar also.
" Output voltage is variable by a factor of 8 from +/- 10V to +/-80V and constantly changes to minimize overall thermal dissipation."

Cheers George

40 amps of current is needed to produce 40 volts peak (100 watts RMS) across a 1 ohm dip in impedance. If this impedance is reactive the amplifier will be really unhappy.

I don't understand it.  40V across 1 ohm (or 40amps thru 1 ohm) would make 1600W. 

Note: Watts RMS means something different and has no relevance.  Power, obtained by multiplying  RMS current and RMS voltage is AVERAGE power (equal half of peak power for sinewave).  It represents DC power that would produce the same amount of heat.  RMS Power does not represent anything useful.

AHB2 power supply is highly regulated SMPS capable of producing large currents.  As for amp being less stable in bridged mode - AHB2 is designed to be stable since it does not have feedback in traditional sense.  Of course in order to reduce distortion that low it has to have negative feedback, but this feedback in not recursive, meaning it is not zapped back to the front of the amp.  Instead it drives another amp  (error amplifier), with another set of power transistors, that at the very end corrects output signal.

Also, low output impedance, doubled in bridged mode, might be important for driving complex loads, but it is already very low, about 0.03ohm @1kHz.  For the purpose of damping the speaker membrane it does not matter, since most of the speaker resistive impedance is in series anyway, reducing highest obtainable effective DF to 1.5

I don't understand it. 40V across 1 ohm (or 40amps thru 1 ohm) would make 1600W.  

 Kijanki,

Thank you. I am glad someone is paying attention.. You are correct it would make 1600 watts. My bad. 

I appreciate your bringing up the speaker resistive issue which indeed lowers the effective damping factor. Please keep reminding people and good luck getting that point across. From what I read it appears people think damping somehow "controls" the speaker. We know the series resistance limits that control. 

If we put aside the term "damping" , because thats not what really matters, we need to inform people that an amplifier with high output impedance may cause a marked change in the frequency reponse of their speaker. Thats what really matters.
 
I invite you to the task at hand.